Factors controlling the development of B-lymphocyte precursor cells from hemopoietic stem cells will be analysed using a combination of cloning assays in vivo (for multipotential, spleen colony-forming cells, CFU-S) and in vitro (for multipotential colony-forming cells and B-lymphocyte colony-forming cells, BL-CFC). Spleen colonies generated by fetal liver or adult marrow CFU-S in irradiated CBA and CBA T6T6 recipients will be recloned to analyse their content of BL-CFC and the specificity of these BL-CFC. Modifications of the in vitro cloning technique for multipotential cells will be investigated to develop a cloning system capable of generating BL-CFC in vitro. Analysis will be made of the mechanism by which purified bacterial cell wall components stimulate the formation of BL-CFC in the spleen and the role played in this process by the serum factor elicited by these bacterial cell wall components. The serum factor will be partially purified and its action in vivo and in vitro on BL-CFC characterized. Studies will be completed on the response of BL-CFC in mice bearing transplanted B-lymphoid tumors.